Drag-inducing stabilizer plates with damping apertures

ABSTRACT

A floating vessel is equipped with perforated plates which exhibit both an added-mass effect and a damping effect. The addition of porosity to an added mass plate phase-shifts the added mass force so that it becomes at least partially a damping force which does not depend on large velocities to develop a large damping force. Preferred porosity is in the range of about 5% to about 15% of total plate area. A semi-submersible drilling rig may have damper plates fitted between its surface-piercing columns and/or extending from the sides of its pontoons. A truss spar offshore platform may have damper plates installed within its truss structure intermediate its hull and ballast tank. Drill ships and similar vessels may be equipped with damper plates extending from the sides of their hulls to reduce both heave and roll. In certain embodiments, the damper plates are retractable so as not to interfere with docking and to reduce drag while the vessel is underway.

CROSS-REFERENCE TO RELATED APPLICATIONS

None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to offshore platforms and vessels. Moreparticularly, it relates to floating structures which employ porous,added-mass stabilizer plates for motion suppression.

2. Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98

U.S. Pat. No. 3,986,471 describes an apparatus for damping verticalmovement of a semi-submersible vessel having submerged pontoons and asmall waterplane area which comprises a submerged damper plate equippedwith valves for providing substantially greater resistance to upwardmovement of the plate than downward movement. The damper plate issupported deep beneath the semi-submersible vessel by flexible,tensioned supports such as chains or cables, at a depth beneath thewater surface in the semi-submerged condition of the vessel where theamplitude of subsurface wave motion is less than the maximum heaveamplitude which would be experienced by the semi-submersible vesselalone under identical sea conditions. The area of the damper plate isseveral times larger than the waterplane area of the vessel. An upwardonly-damping action is achieved due to the entrainment of large apparentmasses of relatively still water by the damper plate.

U.S. Pat. No. 5,038,702 describes a semi-submersible platform supportedon columns with pontoons extending between and outboard of the columns.Damper plates are provided by flat surfaces either on top of theoutboard section of the pontoons or by plates positioned on the columnsabove the pontoons to provide heave and pitch stabilization and motionphase control in relation to the wave action such that when the platformis in the drilling mode, the heave phase of the platform isapproximately 180° out of phase with wave action, and in the survivalmode, heave action of the platform is substantially in phase with waveaction.

U.S. Pat. No. 6,652,192 describes a heave-suppressed, floating offshoredrilling and production platform that comprises vertical columns,lateral trusses connecting adjacent columns, a deep-submerged horizontalplate supported from the bottom of the columns by vertical truss legs,and a topside deck supported by the columns. The lateral trusses connectadjacent columns near their lower end to enhance the structuralintegrity of the platform. During the launch of the platform and towingin relatively shallow water, the truss legs are stowed in shafts withineach column, and the plate is carried just below the lower ends of thecolumns. After the platform has been floated to the deep water drillingand production site, the truss legs are lowered from the column shaftsto lower the plate to a deep draft for reducing the effect of waveforces and to provide heave and vertical motion resistance to theplatform. Water in the column shafts is then removed for buoyantlylifting the platform so that the deck is at the desired elevation abovethe water surface.

U.S. Patent Publication No. 2002/0139286 describes a heave-dampedfloating structure that includes an elongate caisson hull and a plateset coupled to the hull. The plate set includes multiple heave plateslocated about an outer edge of the hull so as to form a discontinuouspattern generally symmetric about a vertical axis of the hull.

BRIEF SUMMARY OF THE INVENTION

The addition of porosity to an added mass plate phase-shifts the addedmass force so that it becomes at least partially a damping force. Thiseffect can develop fairly large damping forces without the need for thelarge relative velocities that drag damping forces typically require. Adamper plate according to the invention can be configured to present alow profile to current forces thereby reducing station-keeping forces inhigh currents.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a perspective view of a battered column, semi-submersibledrilling rig equipped with added-mass stabilizer plates according to afirst embodiment of the invention.

FIG. 2 is a perspective view of a stabilizer plate having slot-typedamping apertures.

FIG. 3 is a perspective view of a stabilizer plate having generallysquare damping apertures.

FIG. 4 is a perspective view of a stabilizer plate having roundhole-type damping apertures.

FIGS. 5A and 5B are cross-sectional views of alternative embodimentshaving paired stabilizer plates.

FIG. 6 is a perspective view showing stabilizer plates according to theinvention mounted between the columns of a battered-columnsemi-submersible drilling rig without pontoons.

FIG. 7 is a perspective view of a truss spar drilling rig equipped witha stabilizer plate having slot-type damping apertures.

FIG. 8 is a perspective view of the truss portion of the spar shown inFIG. 7 equipped with a stabilizer plate having hole-type dampingapertures.

FIG. 9 is a perspective view of the truss portion of the spar shown inFIG. 7 equipped with a stabilizer plate having square damping apertures.

FIG. 10 is a front view (partially in cross section) of a drilling shipequipped with retractable roll stabilizers having damping apertures.

FIG. 11 is a top view of the drill ship shown in FIG. 10.

FIG. 12 is a front view of a drilling ship equipped with hinged rollstabilizers having damping apertures.

FIG. 13 is a top view of the drill ship shown in FIG. 12.

FIG. 14 is a plan view of a stabilizer plate having slot-type dampingapertures.

FIG. 15 is a plan view of a stabilizer plate having generally squaredamping apertures.

FIG. 16 is a plan view of a stabilizer plate having round hole-typedamping apertures.

DETAILED DESCRIPTION OF THE INVENTION

Floating offshore oil platforms and drilling ships need to limit theirmotions as much as possible in order to conduct uninterrupted drillingand production operations. However, these vessels are subject to motion,particularly in the vertical direction (heave), due to the action ofwaves and swells passing the vessel's location. Accordingly, suchvessels are often designed to have minimal waterplane area so that thevessel's buoyancy is affected as little as possible by wave action.

Increasing the added mass is a technique that has been used for sometime to improve the motion characteristics of floating offshoreplatforms. The more massive an object is, the more resistant it is tomotion in reaction to an applied force (e.g., a passing wave).Semi-submersible drilling rigs are often very large and heavy to takeadvantage of this effect. Whenever a floating object moves in a body ofwater, some of the water must move with the vessel. This “attached”water also has mass and thus “adds” to the apparent mass of the vessel.Certain structures may be designed to maximize this effect. For example,heave plates may be added to offshore platforms and other vessels toincrease their effective mass and thereby increase their resistance toacceleration in the vertical direction. Heave plates are typically flatplates fixed in a horizontal position such that moving the plate in avertical direction presents a large surface area to the surroundingwater. This requires a relatively large mass of water to move with theheave plate thereby adding to the apparent mass (and motion stability)of the vessel.

Additionally, the heave plate provides increased drag in the verticaldirection. Drag is a retarding force exerted on a body as it movesthrough a fluid medium such as water. It is generally comprised of bothviscous and pressure effects. One characteristic of drag forces is thatthe force is proportional to the square of the velocity and thus largedrag forces result from large relative velocities.

Damping is a resistive force to velocity. In a system in an oscillatingcondition (such as motion in waves), damping is any effect, eitherdeliberately engendered or inherent to a system, that tends to reducethe amplitude of oscillations of the oscillatory system. Floatingvessels exhibit a heave natural period (oscillation) when displacedvertically. To avoid potentially damaging resonance, it is desirable todesign a floating vessel such that its heave period is outside the rangeof wave periods likely to be encountered. Dampers act to suppressoscillation and generally provide an opposing force that varies inproportion to the system's displacement from its neutral position orstate and the velocity of the displacement.

Perforated heave plates exhibit another damping effect in addition tothat associated with heave plates of the prior art. The addition ofporosity to an added mass plate creates a phase shift in the added massforce so that the water pressure normally associated with added massforces acts as a damping force. The porosity allows the water to lagbehind the structure—i.e., it continues to flow through the plate afterthe plate stops and reverses direction in oscillatory motion. This isvery significant in that the effect allows the development of largedamping forces without the need for the large displacements andvelocities that would be necessary to develop large damping by dragforces.

The invention may best be understood by reference to certainillustrative embodiments shown in the drawing figures.

A battered-column, semi-submersible drilling rig 10 according to a firstembodiment of the invention is shown in FIG. 1. Deck 16 (upon whichdrilling equipment 18 is mounted) is supported on battered columns 12projecting above the waterline. Buoyancy is provided by columns 12 andpontoons 14 which connect columns 12 and form the perimeter of centralopening 24 through which drill string 22 may pass. The invention mayalso be practiced with conventional semi-submersible rigs—i.e., thosehaving vertical columns. When drilling operations are being conducted,rig 10 is held in position by catenary anchor lines 20 which connect toanchors on the seafloor. The invention may also be practiced withdynamically positioned drilling rigs—floating platforms which maintaintheir position using vectored thrust rather than anchors.

Plate-type heave dampers 26 extend between columns 12 below thewaterline and above pontoons 14. Semi-submersible 10 shown in FIG. 1comprises a pair of dampers 26. Other embodiments may have additionaldamper plates. Those skilled in the art will appreciate that it isdesirable to locate the damper plates symmetrically about the center ofthe vessel.

In other embodiments of the invention (not shown), heave dampers 26 maybe mounted to the vertical sides of pontoons 14. Dampers 26 may bemounted on the interior surface (i.e., within central opening 26),exterior surface or both. Dampers 26 in this configuration may becantilevered or braced as dictated by structural considerations.

FIG. 2 shows damper plate 26 in greater detail. Damper 26 comprisesslotted plate 28 connected to support member 32. Slots 30 provideopenings through which water may flow from the upper surface of plate 28to the lower surface of plate 28 and vice versa. Damper 26 may beconstructed of any suitable material or combination of materials. Oneparticularly preferred material is steel which provides relatively highstrength at relatively low cost and may be worked usingreadily-available tools and equipment.

As shown in the exemplary embodiments of the drawing figures, supportmember 32 is a box beam. Other structures including, but not limited to,tubular members and flanged or un-flanged beams may similarly be used.Support members having a watertight internal cavity may also function asbuoyancy members. It will be appreciated that damper plates according tothe invention may be configured to present a relatively small frontalarea to lateral movement of the vessel thereby minimizing the effects ofcurrents and the station keeping forces necessary to hold the vessel inposition. Low frontal area also is advantageous in reducing drag whenthe vessel is being moved from one location to another.

FIG. 3 shows one alternative damper plate 26′ in detail. Damper 26′comprises perforated plate 34 connected to support member 32. Squareapertures 30 provide openings through which water may flow from theupper surface of plate 34 to the lower surface of plate 34 and viceversa. Damper 26′ may be constructed of any suitable material orcombination of materials. One particularly preferred material is steelwhich provides relatively high strength at relatively low cost.

FIG. 4 shows yet another version of damper plate 26″ in detail. Damperplate 26″ comprises perforated plate 38 connected to support member 32.Round apertures or holes 40 provide openings through which water mayflow from the upper surface of plate 38 to the lower surface of plate 38and vice versa. Damper plate 26″ may be constructed of any suitablematerial or combination of materials. One particularly preferredmaterial is steel which provides relatively high strength at relativelylow cost.

FIG. 5A is a cross-sectional view of a fourth embodiment of a damperaccording to the invention. Paired-plate damper 42 comprises upper plate44 and lower plate 46 both of which are connected to support member 32.As shown in FIG. 5A, holes 40 in upper plate 44 may be axially offsetdistance “O” from corresponding holes 40 in lower plate 46.Alternatively, as illustrated in FIG. 5B, holes 40 in upper plate 44′ ofdamper 42′ may be axially aligned with corresponding holes 40 in lowerplate 46′. By selecting the extent (if any) of the offset “O,” theresistance to the flow of water from the upper surface of damper 42 tothe lower surface of damper 42 (or vice versa) which may occur uponvertical movement of damper 42 may be modified, which may influence thedamping effect.

A battered-column, semi-submersible drilling rig 48 according to anotherembodiment of the invention is shown in FIG. 6. Deck 16 (upon whichdrilling equipment 18 is mounted) is supported on battered columns 12projecting above the waterline. Unlike the embodiment illustrated inFIG. 1, buoyancy is provided solely by columns 12 and there are nopontoons which connect columns 12. Rather, columns 12′ are connected bytruss structure 52. Columns 12′ may have undersea section 50 of greaterdiameter to provide the buoyancy needed to support deck 16 withoutincreasing the waterplane area of columns 12′. Perforated heave dampers26 connect adjacent pairs of battered columns 12 and form the perimeterof central opening 24 through which drill string 22 may pass. Theinvention according to the embodiment of FIG. 6 may also be practicedwith semi-submersible rigs having vertical columns. When drillingoperations are being conducted, rig 48 is held in position by catenaryanchor lines 20 which connect to anchors on or embedded in the seafloor.Alternatively, rig 48 may be dynamically positioned.

A truss spar platform according to the present invention is shown inFIG. 7. Truss spar platform 54 comprises generally cylindrical hull 56,truss structure 58 and ballast tank 60, as shown. Deck 16′ is mounted tothe top of hull 56. Drilling equipment 18 may extend over the side ofdeck 16′ so that drill string 22 may be run to the seafloor.Alternatively, a moon pool may be provided in hull 56 for the drillstring with corresponding openings in the damper and ballast tank.Ballast tank 60 (which may contain solid ballast) is sized andpositioned so as to position the center of gravity of the vessel isbelow its center of buoyancy thereby ensuring its free-floatingstability. The rig may be anchored in position by conventional catenaryanchor lines (not shown).

At one or more points within truss structure 58 intermediate the bottomof hull 56 and the top of ballast tank 60 is heave plate 26. In theembodiment shown in FIG. 7, heave plate 26 comprises a slotted plate.FIG. 8 shows an alternative embodiment wherein heave plate 26″ comprisesa perforated plate with holes. FIG. 9 shows yet another embodiment oftruss structure 58 wherein heave plate 26′ comprises a plate havingsubstantially square apertures.

Another embodiment of the invention is shown in FIG. 10. In thisembodiment, ship-shaped offshore vessel 62 comprising hull 64, deck 65and derrick 66 is equipped with retractable motion dampers 68 which maybe extended from the sides of hull 64 below the waterline of the vessel.Motion dampers 68 may be retracted when the vessel is underway to reducethe drag acting on hull 64 or to permit the vessel to come alongside adock or another vessel, such as a supply ship. Motion dampers 68, whenextended, act to reduce both roll and heave of the vessel. Depending ontheir position relative to the center of the vessel, dampers 68 may alsoact to reduce pitching motions of the vessel.

FIG. 11 is a top view of a portion of the drill ship 62 shown in FIG.10. Motion dampers 68 may swing into retracted position 74 (shown inphantom) by pivoting about pivots 72. As shown in FIG. 10, braces 70 maybe attached between hull 64 and motion damper 68 to increase thestructural rigidity of the extended dampers.

The motion dampers 68 shown in FIG. 11 are of the slotted plate type. Itwill be understood that plates having other aperture shapes (such asthose illustrated in FIGS. 15 and 16) may also be used in the practiceof the invention.

Another embodiment of the invention is shown in FIG. 10. In thisembodiment, drill ship 62′ comprising hull 64, deck 65 and derrick 66 isequipped with folding motion dampers 76 which may be extended from thesides of hull 64 below the waterline of the vessel. Motion dampers 76may be retracted when the vessel is underway to reduce the drag actingon hull 64 or to permit the vessel to come alongside a dock or anothervessel, such as a supply ship. Hinged motion dampers 76, when extended,act to reduce both roll and heave of the vessel. Depending on theirposition relative to the center of the vessel, they may also act toreduce pitching motions of the vessel.

FIG. 13 is a top view of a portion of the drill ship 62′ shown in FIG.12. Motion dampers 76 may be moved into retracted position 78 (shown inphantom) by swinging on hinges 80. Braces (not shown) may be attachedbetween hull 64 and motion dampers 76 to increase the structuralrigidity of the extended dampers.

The motion dampers 76 shown in FIG. 13 are of the slotted plate type. Itwill be understood that plates having other aperture shapes (such asthose illustrated in FIGS. 15 and 16) may also be used in the practiceof the invention.

Damper plates according to the present invention preferably have betweenabout 5% to about 15% porosity—i.e., the openings comprise about 5 to 15percent of the total plate area (exclusive of support members).Particularly preferred is a damper plate having a porosity of about 10%.FIG. 14 is a plan view (to scale) of a slotted plate 28 having slots 30which comprise 10% of the plate area. FIG. 15 is a plan view (also toscale) of a perforated plate 34 according to the invention which hassubstantially square apertures in a linear row-and-column configurationwhich comprise 10% of the plate area. FIG. 16 is a plan view (to scale)of a damper plate 40 according to the invention having holes (roundapertures) 40 in a linear row-and-column configuration which comprise10% of the plate area. It will be understood that other apertureconfigurations are also possible and may be employed without departingfrom the scope of the invention. Particularly preferred are apertureconfigurations which are “screen-like”—i.e., those that have relativelysmaller apertures spaced relatively close together as opposed toconfigurations having fewer and larger spaced-apart openings (eventhough the total porosity may be equal).

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe scope and spirit of the invention as described and defined in thefollowing claims.

1. A semisubmersible comprising: a plurality of surface-piercingcolumns; a deck supported on the columns; at least one substantiallyhorizontal, perforated plate having a porosity between about 5 percentand about 15 percent connected to and extending between two columnsbelow the waterline.
 2. A semisubmersible as recited in claim 1 furthercomprising a pair of opposed support members attached to at least onecolumn and the sides of the perforated plate.
 3. A semisubmersible asrecited in claim 2 wherein the support members have an internal cavity.4. A semisubmersible as recited in claim 3 wherein the support membershave positive buoyancy.
 5. A semisubmersible as recited in claim 1wherein the porosity of the perforated plate is about 10 percent.
 6. Asemisubmersible as recited in claim 1 wherein the perforations in theperforated plate comprise slots.
 7. A semisubmersible as recited inclaim 1 wherein the perforations in the perforated plate comprisesubstantially round holes.
 8. A semisubmersible as recited in claim 1wherein the perforations in the perforated plate comprise substantiallysquare apertures.
 9. A semisubmersible as recited in claim 6 whereineach slot in the plate comprises less than about 2 percent of the totalarea of the plate.
 10. A semisubmersible as recited in claim 7 whereineach hole in the plate comprises less than about 0.125 percent of thetotal area of the plate.
 11. A semisubmersible as recited in claim 8wherein each square aperture in the plate comprises less than about0.125 percent of the total area of the plate.
 12. A semisubmersible asrecited in claim 1 wherein the columns are battered columns.
 13. Asemisubmersible comprising: a plurality of surface-piercing columns; adeck supported on the columns; at least one pontoon connected to atleast one column and having an inner side surface and an opposed, outerside surface; at least one substantially horizontal, perforated platehaving a porosity between about 5 percent and about 15 percent attachedto at least one side surface of a pontoon below the waterline.
 14. Asemisubmersible as recited in claim 13 wherein the at least one sidesurface is an inner side surface.
 15. A semisubmersible as recited inclaim 13 wherein the at least one side surface is an outer side surface.16. A semisubmersible as recited in claim 13 further comprising a pairof opposed support members attached to at least one pontoon and thesides of the perforated plate.
 17. A semisubmersible as recited in claim16 wherein the support members have an internal cavity.
 18. Asemisubmersible as recited in claim 17 wherein the support members havepositive buoyancy.
 19. A semisubmersible as recited in claim 13 whereinthe porosity of the perforated plate is about 10 percent.
 20. Asemisubmersible as recited in claim 13 wherein the perforations in theperforated plate comprise slots.
 21. A semisubmersible as recited inclaim 13 wherein the perforations in the perforated plate comprisesubstantially round holes.
 22. A semisubmersible as recited in claim 13wherein the perforations in the perforated plate comprise substantiallysquare apertures.
 23. A semisubmersible as recited in claim 20 whereineach slot in the plate comprises less than about 2 percent of the totalarea of the plate.
 24. A semisubmersible as recited in claim 21 whereineach hole in the plate comprises less than about 0.125 percent of thetotal area of the plate.
 25. A semisubmersible as recited in claim 22wherein each square aperture in the plate comprises less than about0.125 percent of the total area of the plate.
 26. A semisubmersible asrecited in claim 13 wherein the columns are battered columns.
 27. Asemisubmersible as recited in claim 13 further comprising at least onebrace having a first end connected to a pontoon and an opposed secondend connected to the perforated plate.
 28. A semisubmersible as recitedin claim 16 further comprising at least one brace having a first endconnected to a pontoon and an opposed second end connected to thesupport member.
 29. A truss spar comprising: a substantiallycylindrical, surface piercing hull having an upper end and a lower end;a deck supported on the upper end of the hull; a subsea ballast tank forsolid ballast; a truss structure connected at a first end to the lowerend of the hull and connected at a second end to the subsea ballasttank; at least one, substantially horizontal, perforated plate having aporosity between about 5 percent and about 15 percent connected to thetruss structure.
 30. A truss spar as recited in claim 29 wherein theperforated plate is connected to the truss structure at a pointintermediate the first end and the second end.
 31. A truss spar asrecited in claim 30 wherein the perforated plate is locatedsubstantially within the confines of the truss structure.
 32. A trussspar as recited in claim 29 further comprising a pair of opposed supportmembers attached to at least one side of the perforated plate and thetruss structure.
 33. A truss spar as recited in claim 29 wherein thesupport members have an internal cavity.
 34. A truss spar as recited inclaim 33 wherein the support members have positive buoyancy.
 35. A trussspar as recited in claim 29 wherein the porosity of the perforated plateis about 10 percent.
 36. A truss spar as recited in claim 29 wherein theperforations in the perforated plate comprise slots.
 37. A truss spar asrecited in claim 29 wherein the perforations in the perforated platecomprise substantially round holes.
 38. A truss spar as recited in claim29 wherein the perforations in the perforated plate comprisesubstantially square apertures.
 39. A truss spar as recited in claim 36wherein each slot in the plate comprises less than about 2 percent ofthe total area of the plate.
 40. A truss spar as recited in claim 37wherein each hole in the plate comprises less than about 0.125 percentof the total area of the plate.
 41. A truss spar as recited in claim 38wherein each square aperture in the plate comprises less than about0.125 percent of the total area of the plate.
 42. A ship-shaped vesselcomprising: a buoyant hull; a deck attached to the hull; at least onesubstantially horizontal, perforated plate having a porosity betweenabout 5 percent and about 15 percent attached to the hull below thewaterline.
 43. A vessel as recited in claim 42 further comprising acavity in the hull and a pivot connected to the plate such that theplate may retracted into the cavity.
 44. A vessel as recited in claim 42further comprising a hinge attached to the hull and to the plate suchthat the plate may move from a first, substantially horizontal positionextending from the side of the hull to a second, substantially verticalposition where the plate is substantially adjacent and parallel to theside of the hull.
 45. A vessel as recited in claim 42 further comprisinga pair of opposed support members attached to at least one side of theperforated plate and the truss structure.
 46. A vessel as recited inclaim 42 wherein the support members have an internal cavity.
 47. Avessel as recited in claim 46 wherein the support members have positivebuoyancy.
 48. A vessel as recited in claim 42 wherein the porosity ofthe perforated plate is about 10 percent.
 49. A vessel as recited inclaim 42 wherein the perforations in the perforated plate compriseslots.
 50. A vessel as recited in claim 42 wherein the perforations inthe perforated plate comprise substantially round holes.
 51. A vessel asrecited in claim 42 wherein the perforations in the perforated platecomprise substantially square apertures.
 52. A vessel as recited inclaim 49 wherein each slot in the plate comprises less than about 2percent of the total area of the plate.
 53. A vessel as recited in claim50 wherein each hole in the plate comprises less than about 0.125percent of the total area of the plate.
 54. A vessel as recited in claim51 wherein each square aperture in the plate comprises less than about0.125 percent of the total area of the plate.